Method for double-sided patterning of high temperature superconducting circuits

ABSTRACT

In a process for patterning both sides of a double-sided HTS thin film wafer with the patterns in close registration, the first side is patterned with at least one reference mark and the second side is patterned with at least one aperture which permits alignment of the reference mark from the already applied pattern on the first side preferably to a similar reference mark on the yet to be applied patterns on the second side, such that the patterns can be aligned in close registration, using microcopic viewing techniques if necessary.

This application claims the benefit of Provisional application No.60/093,688 filed Jul. 22, 1998.

BACKGROUND OF INVENTION

This invention pertains to a method for applying complex patterns inclose alignment with each other on both sides of a single hightemperature superconducting (HTS) wafer.

In common practice in the art, a HTS thin film is deposited upon one orboth sides of a thin substrate wafer. For most applications today, onlyone device is fabricated on a single wafer; that is, only one side ofthe wafer receives a pattern. The reverse side of the wafer, if employedat all, is employed as a heat sink or other structure that has nostrongly preferred geometric orientation with respect to the pattern onthe first side.

There is increasing interest in applications wherein it is desirable tocouple two superconducting electronic devices electromagnetically atmicrowave or radio frequencies. Such applications include microwavepower transmission, bandwidth and bandpass filters, and imaging, such asmagnetic resonance imaging (MRI). In such applications, there is aconsiderable benefit to laying down the devices to be coupled onopposite sides of the same wafer. In addition, the patterns on the twosides often need to be aligned to micron scale in order to achieveoptimum performance.

While the substrates commonly employed in the art, such as LaAlO₃,sapphire, MgO, NdGaO₃ and yttria stablized zirconia are themselvestransparent at visible wavelengths, the HTS thin films, such asTl₂Ba₂CaCu₂O₈ or YBa₂Cu₃O_(7−x) are opaque at all practical wavelengths.The result is that the first patterned side is not visible whileapplying the pattern to the opposite side, which makes registration ofthe two patterns extremely difficult. Two-sided patterned waters aredisclosed in Shen, U.S. Pat. No. 5,750,473. Until now, the best knownmethod in the art for fabricating such a device is represented by themodel MA6 Mask Aligner manufactured by Karl Suss K G GmbH & Co., Munich,Germany. The MA6 is a complicated apparatus equipped with both bottomside and top side microscopes and image storage capabilities employed topermit simultaneous viewing of the two sides of the opaque wafer. Themajor drawback for an apparatus such as the MA6 is that no matter whatequipment configuration is employed, there will be some device geometrywhich requires placement of reference marks which will be inaccessibleto the optical configuration in place in any device such as the MA6. Inmost instances, the optical configuration could be changed toaccommodate the new sample geometry, but this is an expensive and timeconsuming undertaking at best. Thus, the art does not provide a flexiblemeans to achieve double-sided patterning of HTS wafers of arbitrarygeometry aligned with micron scale precision.

SUMMARY OF THE INVENTION

The present invention provides for a method for providing alignmentreference marks on opposing sides of a double-sided high temperaturesuperconducting wafer comprising the steps of:

applying at least one reference mark to a first high temperaturesuperconducting thin film on a first side of a transparent wafersubstrate;

patterning a second high temperature superconducting thin film on asecond side of the wafer to provide at least one window aperture throughwhich said at least one reference mark is visible.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1-4 provide a schematic illustration of the method of theinvention, in which:

FIG. 1 shows the patterned first side of an HTS wafer with referencemarks;

FIG. 2 shows the unpatterned second side after the etching of windowstherein at the approximate location of the first side reference marks;

FIG. 3 shows the initial positioning of the photomask carrying thepattern to be applied to the second side of the wafer, the photomaskhaving reference marks not fully aligned with those on the first side;and

FIG. 4 shows the final positioning of the photomask after precision ofalignment of the photomask reference marks with the first side referencemarks visible through the second side windows.

FIGS. 5a-5 c depict respectively the first side pattern, the windowapertures and the second side patterns for use in a magnetic resonanceimaging device.

FIGS. 6a-6 c depict respectively the first side pattern, the windowapertures and the second side patterns for use in a nuclear magneticresonance spectrometer.

FIGS. 7a-7 c depict respectively the first side pattern, the windowapertures and the second side patterns for use in a microwavetransmission power filter.

DETAILED DESCRIPTION OF THE INVENTION

This invention is directed to a method for double-sided patterning oftwo sided HTS thin film wafers.

The key step in the fabrication of double-sided patterned devices onwafers is to etch transparent “windows” in the wafer. The windowopenings reveal an alignment mark disposed on the already appliedpattern on the front side that defines the alignment registration forthe second pattern to be applied to the second side. Precision alignmentcontrols allow the photomask for the second side to be preciselyoverlaid with respect to the first side reference marks. Using opticalmicroscopy to guide the alignment, precision between the front and backsurfaces is better than 5 micrometers.

Suitable substrates for the practice of the present invention are thosethat are transparent in the visible or near infrared portion of theelectromagnetic spectrum. Most preferred are substrates that aretransparent in the visible region. Of course, the substrate must bereceptive to the deposition of high temperature superconducting (HTS)thin film. Particularly preferred substrates include LaAlO₃, sapphire,MgO, NdGaO₃, yttria stablized zirconia, quartz and strontium titanate,for example.

There is no particular limitation on the shape of the substrate.Typically, the substrates are in the shape of thin, round wafersapproximately 500 micrometers in thickness and 5.08 or 7.62 centimeters(2 inch or 3 inch) in diameter.

The process of this invention may be practiced with any known HTS thinfilm material. Such HTS materials include Tl₂Ba₂CaCu₂O₈, YBa₂Cu₃O_(7−x);(Tl, Pb)Sr₂Ca₂Cu₃O₉, BiPbSrCaCuO, BiSrCaCuO, BiSrCuO, BiSrYCuO andothers.

There is no limit in principle to the manner of deposition of the HTS onthe substrate and any of the known, commonly used methods, such asoff-axis magnetron sputtering (in situ or ex situ, the so called“two-step process”, laser ablation, etc.) may be employed. Off axismagnetron sputtering and laser ablation are particularly preferred. Itis not strictly necessary to deposit the HTS film on both sides at onetime, but it is preferred. The thin film is typically deposited to athickness of about 0.4 to 0.7 microns, but that is not critical to thepractice of the invention.

The HTS thin films such as are known in the art, are opaque in thevisible and near-infrared portions of the spectrum. The underlyingprinciple of the present invention is that reference marks on a firstpatterned side of a wafer can be made visible by opening windows on thesecond, not-yet-patterned side in approximate registration with thereference marks on the first side. Once the windows are open, theapplication of the second pattern may proceed in precise alignment withthe now visible reference marks on the first pattern.

Any convenient means of applying the reference marks to the first sideof the HTS thin film wafer is suitable, and the practice of the presentinvention is not limited to any particular means. A preferred method isto create the reference marks as part of the photomask design for thefirst pattern to be applied. However the reference marks may be appliedby numerous other means. the only criterion being that they must beclearly observable from the other side after the windows are “opened.”The number of reference marks and their distribution on the first sideof the wafer is not limited in any way except that they must be appliedto avoid interference with the performance of the thin film HTS devicedeposited. For instance, it is customary in preparing wafers to leave agap of about 2 mm around the periphery of the substrate where no HTSfilm is deposited. The reference marks can be conveniently located inthis gap so as not to interfere with the pattern of interest.Preferably, there are two reference marks, but there could be more; and,one will sometimes suffice.

The actual process of patterning the HTS thin films is well-known in theart and thus need not be explained in great detail. Essentially aphotoresist, such as poly(methyl methacrylate), is applied to the HTSthin film by any convenient coating technique such a spin coating. Anegative image of the desired pattern is made in a photomask which isthen overlaid onto the photoresist and the photoresist layer is exposedor irradiated through the photomask, causing the photoresist topolymerize and harden in the exposed areas. The photoresist layer isthen developed to remove the unexposed (unpolymerized) areas and theunderlying HTS film is etched by, e.g., ion beam etching, to create apattern in the HTS film. The polymerized areas of the photoresist layerare then removed, leaving the patterned HTS film.

In the preferred practice of the invention, the reference marks areincorporated into the pattern. However, it should be understood thatother means could be employed to apply the reference marks. For example,reference marks could be created by physically notching the substrate.Such would need to be done after the HTS films are annealed, however,otherwise there is a good probability that the substrate will be damagedduring firing.

After the first side of the double-sided wafer is patterned, theopposite side is patterned in the same manner. The principle differencebeing, that the second side is patterned with windows which, when“opened” will reveal the reference marks from the first side. In thepreferred practice of the invention, the opposite or second side of thewafer is patterned in phases. In the first phase, a photomask consistingonly of the windows, is prepared and applied over the photoresist. Thephotoresist is then exposed, developed and stripped as on the first sideafter etching the HTS film. These process steps open the windows. Whilethe windows need not be any particular size, they should be large enoughto permit viewing of the reference marks with unaided visual alignmentof the photomask to the patterned side. In addition, the size of thewindow is a function of the tolerance of the equipment used to align thepatterns. The higher the degree of precision on the equipment, thesmaller the windows can be. Generally speaking, a square-shaped windowof approximately 0.5 mm per side is sufficient.

Once the windows are opened and the reference marks from the first sideare visible, another photoresist layer is applied and another photomaskcontaining the desired circuitry pattern is applied to the resist layer.Before application of the pattern to the second side, the referencemarks of the first side, visible through the windows, and the referencemarks on the photomask containing the desired pattern of circuitry forthe second side are brought into precise alignment. Once the alignmentof the reference marks and/or windows has been made the second side isexposed, developed, etched and stripped as above, leaving a wafer havingtwo patterned HTS films in close registration with one another.

In order to achieve high precision alignment, it is preferred toobserved and manipulate the alignment of the reference marks under someform of magnification. A preferred method of magnification is to use avibration isolation table and 50× or 100× magnification (10× eyepieceand either 5× or 10× optics).

While not required for the practice of the present invention, it isfound that for practical reasons, it is desirable to apply thephotopolymer to both sides of the HTS wafer in order to protect the sidewhich isn't undergoing patterning from physical damage in handling.

With reference being made to FIGS. 1-4, the process as generallydescribed above is illustrated schematically. It should be noted thatfor the purpose of illustration the reference marks aredisproportionately large compared with the patterns. It is anticipatedthat in most cases of practical interest, the reference marks will bemuch smaller compared to the pattern, and will likely requiremagnification of about 50-100× to achieve proper alignment.

In FIG. 1, the first side of the HTS-thin-film-coated wafer 1, has beenpatterned using a first photomask, and both device 2, and referencemarks 3, have been applied. In FIG. 2, the second side of theHTS-thin-film-coated wafer 4, has been patterned with windows 5, using asecond photomask, the windows being imperfectly aligned with thefirst-side reference marks, making visible the first side referencemarks. In FIG. 3, a third photomask 6, having both the device pattern 7,to be applied to the second side of the HTS-thin-film-coated wafer, andreference marks 8, to provide alignment with the device pattern on thefirst side, is positioned in imperfect alignment with the pattern on thefirst side. In FIG. 4, the third photomask is shown in precise alignmentwith the pattern on the first side as a result of bringing the referencemarks into precise alignment.

EXAMPLES Example 1

A 0.7 micrometer thick film of Tl₂Ba₂CaCu₂O₈ was deposited usingoff-axis magnetron sputtering and a standard two-step post annealprocess on a both sides of a 0.5 mm thick LaAlO₃ wafer 5.08 cm indiameter. Using a Model PM101DT Headway Spinner, a solution of apoly(methyl methacrylate) photoresist, 9% solids in chorobenzene(available from OCG-Olin Chemical Corporation), was spun coated ontoboth the front and back surfaces of the wafer to coating thickness of1.2 micrometers. The photoresist was then baked at 170° C. for 30minutes in a Model LC-02-W Lindberg Blue-M Oven in a nitrogenatmosphere. A 1.8 micrometer layer of Hoechst AZ 5214 positive resistwas spin coated as above onto the photoresist layer on one side of thewafer. After a 90° C. post-bake in the Lindberg Blue-M Oven, the AZ 5214positive resist was exposed to UV light in the range of 137.5 mJ persquare cm through a photomask pattern and incorporating reference marksas shown in FIG. 5a. The exposed AZ 5214 photoresist was then developedby immersion in ion free AZ 422MIF developer Hoechst) for 1 minute and40 seconds.

After exposure and development of the AZ 5214 resist, the wafer wassubjected to an oxygen plasma using a YES-CV100PZ Downstream OxygenPlasma Stripper (Yield Engineering Systems. Inc.) operating at 500 Wpower and 160 Pascal (1.2 Torr) pressure at 90° C. for 20 seconds todescum the wafer. The pattern was then transferred to the poly(methylmethacrylate) photoresist by flood exposure to deep (220-260 nm) UVlight using a 500 watt, Model 83210 deep UV light source (OrielCorporation) at 10 J per square centimeter. The resist layer was thendevelopment in toluene at 70° C. for 4.5 minutes. The HTS film was thenetched by argon ion beam milling using a Model LL-250 Microtech (VeecoInstruments). The Ar+ ion beam energy was 500 eV and the beam currentwas 500 mA. The beam was directed along the film normal. During milling,the wafer was cooled from behind by a helium coolant gas. The residualphotoresist was removed with an oxygen plasma using the Plasrha Strippermentioned above at 150° C. for 3 minutes. This completed the patterningof the front side.

The opposite side of the wafer was then patterned as follows. A maskconsisting of a single window as shown in FIG. 5b was employed and,using the techniques described above, the window was opened to permitviewing of the reference mark on the opposite side of the wafer. Thenanother photomask having the pattern shown in FIG. 5c was used and wasaligned to the pattern already applied to the front side of the waferusing a model MA6 Mask Aligner. Subsequent to the alignment, theexposure, development, and etching procedures described above were usedto complete the patterning of the second side of the wafer and thuscreate a double-sided HTS wafer patterned on both sides.

Example 2

The procedures of Example 1 were exactly replicated except that a 0.6micrometer thick film of YBa₂Cu₃O₇ was deposited on both sides of a 0.5mm thick sapphire wafer 5.08 cm in diameter and the photomasks were asshown in 6 a-6 c.

Example 3

The procedures of Example 1 were exactly replicated except that a 0.7micrometer thick film of Tl₂Ba₂CaCu₂O₈ was deposited on both sides of a0.5 mm thick sapphire wafer 5.08 cm in diameter. and the photomasks wereas shown in FIGS. 7a-7 c.

What is claimed is:
 1. A method of aligning a pattern on a side of adouble-sided high temperature superconducting wafer comprising the stepsof: applying at least one reference mark to a first high temperaturesuperconducting thin film on a first side of a transparent wafersubstrate; opening a window aperture in a second high temperaturesuperconducting thin film on a second side of said wafer through whichsaid at least one reference mark is visible; and aligning a pattern onthe second side of the wafer according to the at least one referencemark.
 2. The method of claim 1 further comprising patterning at leastone side of said wafer to form an electronic device.
 3. The method ofclaim 2, further comprising sequentially patterning both sides of saidwafer to form an electronic device on each.
 4. The method of claim 3wherein the second pattern to be applied is brought into precisealignment with the first applied pattern by alignment with said at leastone reference mark prior to the step of applying the second pattern ontothe high temperature superconducting wafer.
 5. The method of claim 2,wherein the at least one reference mark and the pattern of theelectronic device are simultaneously applied to the first side of thewafer.
 6. The method of claim 1, wherein said aligning step comprises:a) applying a photoresist to said second high temperature superconductorthin film; b) applying a photomask to said photoresist, said photomaskconsisting solely of at least one window aperture approximatelycorresponding in location to the at least one reference mark on thefirst high temperature superconductor thin film; c) irradiating thephotoresist layer through said photomask to polymerize said photoresistin the exposed areas; d) developing the photoresist to removeunpolymerized areas corresponding to the at least one window aperture;and e) etching the second high temperature film to create a windowaperture therein, to reveal the at least one reference mark on the firsthigh temperature superconductor thin film.
 7. The method of claims 2 or3 wherein the step of patterning comprises the steps of: a) applying aphotoresist to said first high temperature superconductor thin film; b)applying a photomask to said photoresist; c) irradiating the photoresistlayer through said photomask to polymerize said photoresist in theexposed areas; d) developing the photoresist to remove unpolymerizedareas; e) etching the first high temperature film; and f) removing thepolymerized areas of the photoresist.
 8. The method of claim 1, whereinthe transparent substrate is selected from the group consisting ofinclude LaAlO₃, sapphire, MgO, NdGaO₃, yttria stablized zirconia, quartzand strontium titanate.
 9. The method of claim 1, wherein the first andsecond high temperature superconductor thin films are selected from thegroup consisting of Tl₂Ba₂CaCu₂O₈, YBa₂Cu₃O_(7−x), (Tl, Pb)Sr₂Ca₂Cu₃O₉,BiPbSrCaCuO, BiSrCaCuO, BiSrCuO and BiSrYCuO.